[libc++] Fixes concepts overload resolution.

D123182 fixes a bug in Clang's overload resolution. After it landed it
was discovered `basic_format_arg`'s constructors contains this bug. This
fixes the bug in libc++, unblocking D123182.

The code has been tested in combination with D123182.

Reviewed By: royjacobson, #libc

Differential Revision: https://reviews.llvm.org/D124103